A manhole cover is traditionally formed as a massive cast-iron disk that is received in a support rim embedded in a surface, such as a roadway. As such a cover is extremely heavy it inherently remains well seated on its rim, even when relatively large vehicles pass over it and subject it to considerable vibration.
Recently such cast-iron mahhole cover disks have been replaced with substantially lighter malleable iron steel, or spherolytic (globular) graphite disks which can be produced at considerably lower cost, are much easier to handle, and are physically as strong as the old-fashioned cast-iron disks. Such relatively light disks do pose a difficult problem in that it is necessary positively to lock them in place on the respective rims. Otherwise when subjected to the considerable vibration and external forces to which such a cover is inherently subjected, such manhole covers can come loose, uncovering the relatively large pit underneath them and presenting a considerable danger to traffic.
The commonest method of locking such a manhole cover disk to its rim is simply to provide the disk with a plurality of radially extending bayonet-type projections. The room is correspondingly formed with a plurality of inwardly open recesses so that the cover can be placed on the rim with the projections lying at the recesses, and then rotated to lock the two members together bayonet-fashion. It has also been suggested to provide individual physical latching members that extend from the cover into correspondingly shaped recesses in holes in the rim, and to use a tool or key for relatively rotating the latching members and the hole in the rim so as to lock them together.
Both of these arrangements have the cosiderable disadvantage that the maintenance personnel, after having worked in the manhole pit and replaced the cover, frequently forget to lock it properly in place. Once positioned over the rim there is no way to tell whether the cover is locked in place without trying to pry it up. Thus such a relatively light cover is frequently left merely lying on its rim so that vibration can loosen it and present the above-mentioned traffic hazard.